Yes, as long as you dont have 2 networks on the same page like link-vault and co-op your fine. You can have co-op just on your forums and LV on your frontpage for example.
I read somewhere probably on faqs that you cannot set coop on subdomains.It means it can only be set on main domain ? Help me understanding this point.
Yes it make sense because subdomain can be a completely separate websie .So can we register multiple accounts on same domain but on different domain names? If yes then how pages indexed would be considered?
You'd lose all your indexed pages for duplicate content if you pointed loads of domains at the same site, which would then leave you with no coop weight defeating the point.